Mr. Assaf is grumpy because, like other pilots who ply the skies over South America's Amazon basin, he's been coughing through ashen skies for six weeks, since the start of the 1991 burning season.

Thousands of ranchers are again setting the forest ablaze to create new cattle pastureland, and the jungle is ablaze in a vast arc stretching from near the Amazon's Atlantic mouth south and west 3,000 miles to the border with Peru. The smoke is so dense that dozens of airports in five Brazilian states have closed for lack of visibility.

The burning will last through October, but already the smoke suggests that terrible devastation is under way, after two years in which environmentalists were heartened to see the Amazon burn-off diminish.

"The burnings and deforestation this year have reached very high levels," said Domingo Ribeiros, enforcement chief for Brazil's Environmental Protection Agency, in an interview in Brasilia.

For the world, the Brazilian destruction is part of an increasingly discouraging global picture; a recent United Nations study reported that the world's rain forests are being destroyed much faster than environmentalists had feared.

For Brazil, the fires are a sign that President Fernando Collor de Mello has failed to articulate a clear Amazon policy or to balance the tensions between Brazil's development needs and conservation imperatives.

Last year, Jose Lutzenberger, Mr. Collor de Mello's environment secretary, canceled several subsidies that had encouraged ranchers to destroy forests. But tax loopholes that leave intact the profits of speculators who buy, clear and resell forest lands continue to encourage the devastation, according to Philip Fearnside, an American expert on deforestation who works at the National Institute for Amazon Studies in Manaus.

Mr. Collor de Mello's government is divided between Mr. Lutzenberger's conservationists and rightists representing loggers and ranchers.

Gen. Thaumaturgo Sotero Vaz, an army officer, said recently that the international ecology movement posed a mounting threat to Brazil's sovereign right to develop the Amazon and warned ecologists: "We're going to attack them like guerrillas."

With Brazil's Amazon policy a muddle, it has fallen to the environmental protection agency Mr. Lutzenberger heads, Ibama, to limit the destruction by deploying forest rangers in a yearly campaign of patrolling and fines.

The Ibama rangers don't fight fires -- once ignited, forests are allowed to burn freely. And because it is not illegal to clear privately owned forest, Ibama can fine only those who burn forests without the required permit.

Furthermore, Ibama is underfunded, underequipped, understaffed and overwhelmed. When in early August 500 Ibama rangers began their effort to patrol the Amazon basin, they had just six helicopters.

Nonetheless, Ibama has succeeded in focusing public attention on the most outrageous environmental crimes. Last year, for instance, rangers fined a rancher who defoliated 4,500 acres of virgin timber on his 57,000-acre fazenda by spraying a deadly herbicide from a crop-duster. Weeks later, the agency fined a logging company found to be hiding enough illegally cut hardwood to load 600 flatbed trucks in an underwater deposit in the Amazon River.

Ibama officials say only about 20 percent of the fines are paid, but some ranchers still put up fierce resistance. On Aug. 20, an Ibama helicopter on patrol took ground fire from an angry rancher, forcing an emergency landing, Mr. Ribeiros said.

Despite the difficulties, the government has credited Ibama with reducing destruction in 1989 and 1990, after ranchers burned a forest area the size of Massachusetts in 1988. But Mr. Fearnside said the heavy rains of the past two years have contributed more than Ibama's patrolling to the recent declines.

"Deforestation is still raging out of control," Mr. Fearnside wrote recently.

The crisis was obvious this year around Juara, a town of 30,000 squatting on the southern edge of the rain forest 1,400 miles west of Rio de Janeiro in Mato Grosso state. After this year's immense smoke cloud forced the closure of regional airports, Ibama officials on Aug. 24 banned all further burning in the states of Mato Grosso, Rondonia and Acre.

But when two reporters arrived two weeks later, forest plots all around Juara were on fire, the town's dirt streets were engulfed in smoke, and Tadeu Sales, Ibama's lone agent in the 14,000 square miles of surrounding countryside -- a jurisdiction one-fourth the size of Florida -- said he hadn't heard of the burning ban.

Although Mr. Sales had issued no fines for forest burning, he said he had received 175 requests for burning permits and had approved all but two.

Mr. Sales drove reporters north along a dirt road where virgin forest alternated with wasteland still smoking from recent blazes. The southbound traffic was a stream of lumber trucks groaning under the weight of mahogany logs, chain-sawed to the ground in the first stage of Amazon clearing that culminates in burn-off. Mr. Sales said he had no way of knowing if the trees had been cut legally.

Back in Juara, Celso Azoia, a sawmill owner, said he bought 30,000 acres of forest near Juara in 1984 and had logged off 40 percent of it in the years since. Watching as workers winched a 400-year-old mahogany trunk sideways into a band saw, Mr. Azoia guffawed at the suggestion that the Amazon is in danger.

Across town, Jocelino de Oliveira Neto, a rancher who has cleared 6,000 of 38,000 acres of forest, expressed irritation that the ecology movement had depressed Amazon land prices. Mr. de Oliveira Neto said he planned to clear 15,000 more acres.

Others aren't so enthusiastic. A radio broadcaster, a restaurant manager and a trucker lamented what they consider the rape of the virgin countryside that drew them a decade ago to the Brazilian frontier.

After dark, the burning forest cast a magenta glow on the smoky sky over Juara. The towering trunks of burning Brazil nut trees glowed through the night air as cascades of coals showered to the ashen earth.

Dr. Eulalia Rech, a physician visiting Juara for the first time, expressed disgust at the landscape of fire and ash.

"I thought I was going to paradise, but I found an inferno," Dr. Rech said.